1. Field of the Invention
The present invention relates to an edge-enhancement device which enhances edges in an image captured by an imaging apparatus such as an endoscope, and an edge-enhancement method for enhancing the edges in an image.
2. Description of the Related Art
Conventionally, an original image signal captured by an image sensor in an imaging apparatus such as an endoscope is subjected to edge enhancement in order to emphasize subject contours in the image, and is then displayed on a monitor as an output image. In edge enhancement, the so-called edge component of each target pixel, generated on the basis of the difference between the luminance of the target pixel and the luminance of the surrounding pixels, is added to the luminance of the target pixel.
When the difference in luminance between the target pixel and the surrounding pixels is large in the original image signal, the luminance of the target pixel becomes large in the output image. Hence, the contours in the image are clearer in the output image.
However, in high-luminance areas of the original image, the luminance value may sometimes be increased to excess by the edge enhancement, causing cat's-eye or halation. Furthermore, in the low-luminance areas of the original image, noise may also be over-emphasized.
Japanese Unexamined Patent Publication (KOKAI) No. 2004-46329 discloses that the edge component of the target pixel is weighted according to the luminance difference between the target pixel and the adjoining pixel, and the weighted edge component is added to the luminance value of the target pixel so to produce an edge-enhanced image. In this edge enhancement, when the luminance difference between the target pixel and the adjoining pixels is small or large, the weight becomes small, but when the difference is medium, the weight becomes large. Hence, the pixels with luminance values which differ greatly from the adjoining pixels are not enhanced strongly, which reduces halation in the high-luminance image areas and reduces noise emphasis in the low-luminance image areas.
However, in the above-mentioned edge enhancement, since the luminance value is weighted according to the luminance difference between the target pixel and the adjoined pixels, when one or more pixel luminance values have false values due to noise, the false luminance value influences not only the same pixel, but also the other surrounding pixel values, which results in an unacceptable increase in noise.
Furthermore, the image is usually rendered in the middle of the luminance value range, but the contour of the subject is not always represented by a medium luminance difference. Therefore, the contours of the subject are not always emphasized appropriately by the above-mentioned edge enhancement.